If the axons that provide the sole or predominant synaptic input to a neuron or to an effector cell are interrupted, the • Fig. 4.10 A, Normal motor neuron innervating a skeletal muscle fiber. B, A motor axon has been severed, and the motor neuron is undergoing chromatolysis. C, This is associated in time with sprouting and, in D, with regeneration of the axon. The excess sprouts degenerate. E, When the target cell is reinnervated, chromatolysis is no longer present.
and even death. The best known example of this is atrophy of skeletal muscle fibers after their innervation by motor neurons has been interrupted. However, if only one or a few of the innervating axons are removed, the other surviving axons may sprout additional terminals, thereby taking up the synaptic space of the damaged axons and increasing their influence on the postsynaptic cell.
In the PNS, after an axon is lost through injury, many neurons can regenerate a new axon. The proximal stump of the damaged axon develops sprouts (see
Fig. 4.10C ), these sprouts elongate, and they grow along the path of the original nerve if this route is available (see
Fig. 4.10D ). The Schwann cells in the distal stump of the nerve not only survive the wallerian degeneration but also proliferate and form rows along the course previously taken by the axons. Growth cones of the sprouting axons find their way along these rows of Schwann cells, and they may eventually reinnervate the original peripheral target structures (see
Fig.
4.10E ). The Schwann cells then remyelinate the axons. The rate of regeneration is limited by the rate of slow axonal transport to about 1 mm/day.
In the CNS, transected axons also sprout. However, proper guidance for the sprouts is lacking, in part because the oligodendroglia do not form a path along which the sprouts can grow. This limitation may be a consequence of the fact that a single oligodendroglial cell myelinates many central axons, whereas a single Schwann cell provides myelin for only a single axon in the periphery. In addition, different chemical signals may affect peripheral and central attempts at regeneration differently. Other obstacles to successful CNS regeneration include formation of a glial scar by astrocytes and lack of trophic influences that guided axonal trajectories during development.
1.
The functions of the nervous system include excitability, sensory detection, information processing, and behavior.
2.
The CNS includes the spinal cord and brain. The brain includes the medulla, pons, cerebellum, midbrain, thalamus, hypothalamus, basal ganglia, and cerebral cortex.
3.
The neuron is the functional unit of the nervous system. Neurons have three major compartments: the dendrites, cell body, and axon. The first two receive and integrate signals, and the axon conveys the output signals of the neuron to other cells.
4.
The PNS includes primary afferent neurons and the sensory receptors they innervate, the axons of somatic motor neurons, and autonomic neurons.
5.
Information is conveyed through neural circuits by action potentials in the axons of neurons and by synaptic transmission between axons and the dendrites and somas of other neurons or between axons and effector cells.
6.
Different types of neurons are specialized as a consequence of their individual morphology and the ion channel distribution in the cell membrane of their soma, dendrites, and axons.
7.